1. Field of the Invention
The invention relates to a reciprocating piston compressor with delivery rate control having a valve lifter, which can be electromagnetically operated in the working cycle and is arranged on at least one of the automatic suction valves, for periodically holding the appropriate suction valve open over a defined crank angle range.
2. The Prior Art
So-called return flow control, with which the at least one suction valve per cylinder is held open over a defined range of the compression stroke, is often called upon for controlling the delivery rate of reciprocating piston compressors which preferably run at constant speed. The pressure forces or flow forces of the gas, which is pushed back via the held-open suction valve, can only close the closing element of the respective suction valve after a certain part of the piston stroke has been overcome, as this closing element is subject to an opposing force from the other side which is set according to the required delivery rate reduction. The greater this opposing force, the later in the compression stroke the respective suction valve closes, whereby the delivery rate falls. As, at some point, if the constant opposing force is set too high, the suction valve no longer closes, with this type of compressor control the control range must be limited in a downwards direction in order to avoid the compressor temporarily running on no load with all the problems associated therewith. With these delivery rate control systems, designs have been disclosed with which the loading device for the suction valve to be held open is simply pre-loaded hydraulically or pneumatically, wherein the delivery rate can be affected by varying the appropriate pre-load pressure.
Furthermore, by way of example, a return flow control for reciprocating piston compressors with which a hydraulic control cylinder, which can be charged with pressure medium and relieved periodically in the stroke cycle by means of a control element, acts on the closing element of the suction valve to be held open in the direction of the stroke, is disclosed in EP 694 693 A1. Here, the hydraulically applied lifting force is suddenly reduced at a defined crank angle, as a result of which a reliable and rapid closing of the suction valve is initiated. Similar delivery rate controls with pneumatic actuation are also disclosed in EP 1 400 692 A1, which has the advantage that the actuation force can be derived directly from the reciprocating piston compressor itself. However, because of the relatively high compressibility of the compressed gas, accurately defined conditions for the volumes to be exhausted and the exhaust times must be maintained.
Furthermore, reciprocating piston compressors with an electromagnetically operated return flow control of the kind mentioned in the introduction have been known for a very long time. For example, from DE 1 251 121 A or DE 849 739 B and similar publications, in some cases dating back to the 1930s, in which a valve lifter acting on the sealing element of the suction valve is moved by means of a solenoid, the periodic excitation of which is carried out, for example, by a collector which turns synchronously with the crankshaft of the compressor. The particular characteristic of the actuating force necessary for holding the suction valve open while the suction gas is being pushed back requires a high force from the magnetic actuator, which, with the simultaneous requirement for low heat development, necessitates rather large solenoids. At the same time, the solenoid must be highly dynamic in order to be able to open and close quickly, which if anything cannot be achieved with large solenoids, as the stored energy and the outlay to build up or dissipate current in large solenoids is considerably greater. Especially with arrangements of the kind mentioned in the introduction, heat expansion and wear can cause a displacement of the necessary working stroke range of the magnetic actuator. As solenoids have a very restricted usable working range of a few millimeters stroke length, it would also be necessary to use correspondingly enlarged solenoids in order to be able to still apply correspondingly high actuating forces when the working stroke range is displaced, which further reduces the actuating dynamics. All this previously conflicted with an electromagnetic return flow control of the kind mentioned in the introduction, especially when higher compressor speeds were required and circumstances did not allow the use of separate cooling systems in the vicinity of necessarily large magnetic actuators.
The object of the present invention is to improve an arrangement of the kind mentioned in the introduction so that the stated disadvantages of the related prior art are avoided and that the required high actuating dynamics can also be provided with low heat losses, particularly with small solenoids as actuators of the valve lifter actuating device.